Electromagnetic radiation from an optical source is directed onto an optical aperture in a metallic structure. The metallic structure in turn emits optical output from an emission region in the structure and onto a recording medium (e.g., a magnetic recording disk), thereby heating the medium. The optical output is enhanced when the electromagnetic radiation from the optical source includes a frequency that matches a waveguide mode resonance in the metallic structure. Features (such as ridges or trenches) in the metallic structure may be used to further increase the emitted optical output beyond what the emitted optical output would be in the absence of these features. The apparatus and associated method are useful for data recording, e.g., thermally assisted data recording.